If your pond is new and the filters are not mature yet don't buy a lot of fish, just buy a few cheaper ones and buy more (about 2 every 4 weeks) when your system has matured (in about 4 weeks).
Make a point to buy one fish that is white. When water conditions deteriorate a white fish will have its fins turn pink or red and the whole fish could blush giving you a visual indicator that something is wrong. You cannot do that with fish whose fins and tails are colored.
When to buy. The best selection of pond fish can be found in the spring although you can find some fish during the winter months it is suggested you do not put them in your outdoor pond as the temperature shock can kill them. If they are small enough keep them in a 20 gallon aquarium at room temperature until the outside ponds temperature more closely matches the aquariums.
Where to buy. It is important to look at the state of the ponds and fish at the shop you are intending to buy from. Make sure the water is clean and doesn't smell. If there is a strong fishy smell and the fish don't look happy it could be worth avoiding that dealer.
How many fish can I have in my pond? As a general rule of thumb, for koi, you should have 1 inch of fish for every 10 square foot of water and they should not be put in a pond smaller than 1,000 gallons. For goldfish, aim for 1 inch of fish for every 3 square feet of water. Plan on the size the fish will grow into since you will not want to give them away once they get too big for the pond. Koi can get up to 30 inches long and gold fish up to 10 inches. Over crowding will result in poor water quality and your fish population having many health problems and deaths. With proper filtration there is more room for fish.
Choosing fish. When you have seen a fish you like spend a bit of time watching it. It should be swimming normally, it shouldn't have jerky movements or have its fins clamped to its body. Make sure it doesn't have gill problems, it shouldn't appear to be breathing hard and its gill plates shouldn't stick out from the head. Also avoid fish with holes or raised scales, or fungus. If you decide you want the fish you can have a closer look at it when the dealer catches it, examine it for any redness or marks, or large parasites such as fish lice or anchor worms. And remember never buy a fish because some dealer tells you what a great fish it is, choose a fish because it is beautiful to you.
Transporting fish. When you have chosen your fish the dealer will bag it for you and fill the bag with oxygen. The oxygen is as important as the water, especially on a long journey, there should be enough water to cover the gills when the bag is on its side and the rest oxygen. fish are usually double bagged to avoid leaks, and should be kept in the dark on the journey, either in a black bin liner or in the boot. It is a good idea to put the bag in a box to minimize movement on the journey, lay the bag on its side so the fish has more room and is less likely to be damaged. When you get home float the bag on the pond for 20 minutes so that the temperature will equalize and then release the fish. Ideally new fish should be kept in a quarantine pond for a few weeks so no disease is introduced to the existing fish, but this is not normally possible for most fish keepers, it might be good to use an anti-parasite treatment to reduce the risk of disease. Do not put fish in a newly completed pond. Wait a couple of weeks for the ecosystem to get going and the chlorine to disapate. If you don't you may end up with dead fish within a week or so.